Bump numpy used for Python 3.9 for macOS ARM

There wasn't a Python 3.9 macOS ARM wheel for numpy 1.20,
but there is for numpy 1.21

Author: peterjc

requirements.txt

@@ -12,7 +12,7 @@
 
 numpy==1.14.5; python_version=='3.7'
 numpy==1.17.3; python_version=='3.8'
-numpy==1.20; python_version=='3.9'
+numpy==1.21.0; python_version=='3.9'
 numpy==1.22.3; python_version=='3.10'
 numpy==1.23.3; python_version=='3.11'
 numpy==1.26.1; python_version=='3.12'